Poor Man’s Space Probe

Poor Man’s Space Probe

Astronomy Through a Microscope

The ALH Meteorite, about the size of a softball and one of more than two dozen Mars samples available for study on Earth today. [ALH84001] was found at Allen Hills, Antarctica. Around 28 Mars meteorites have been identified so far.Image Credit: NASA/ Johnson Space Center

William Blake’s vision: "To see a world in a grain of sand" is being realised by a group of researchers at the University of Manchester. Their work, which involves analysing minute samples of material of extraterrestrial origin, is shedding new light on the origin of our planet and the Solar System. They are among the world-leaders in this seemingly esoteric field.

The samples come from meteorites – fragments of rock that have crashed through the Earth’s atmosphere from space. Most meteorites consist of the debris left over from the formation of the planets, which is why their analysis is important in understanding the origin and evolution of the Solar System, and the Earth’s place within it. In the near future the researchers will have access to even more significant samples – those brought back by space missions to comets and our nearest planetary neighbour Mars.

Although meteorites are sometimes known as ‘the poor man’s space probe’, these remarkable objects require sophisticated (and expensive) instruments to extract their primaeval secrets. The Manchester group has developed new analytical techniques and instruments to study meteorites, which have benefited other areas of study, particularly the earth sciences and materials science.

In a sense they are heirs of the 19th-century Sheffield industrialist Henry Clifton Sorby, who showed that you could understand how mountain ranges develop by looking at slivers of rock through the newly invented petrological microscope. Sorby developed his microscope to study the grains of iron-nickel alloy found in meteorites, and then went on to show how it could be used to understand and improve the properties of Sheffield steel. This must surely be the earliest example of an academic interest in ‘space research’ leading to industrial benefits.

Since then, the study of meteorites has brought together physicists, chemists, geologists and astronomers, and has been a fertile breeding ground for advances that have taken earth sciences into space and brought astronomy into the laboratory.

The first signs of that amalgamation came with the advent of the Apollo Moon landings in 1969. A dozen or so UK research groups contributed to the program, the largest number outside the US. Among them was Grenville Turner, then a young lecturer based at Sheffield University, who developed a new method for dating rocks, called argon-argon dating. Aside from producing the first accurate ages of the lunar surface, it provided the basis for current estimates of the probability of large extraterrestrial bodies striking the Earth. The technique is also now routinely used in the earth sciences and has spawned a mini industry of specialist mass-spectrometer builders, a field in which UK industry still has a leading international role. Mass spectrometers are used to measure abundances of isotopes and can be used to study a wide range of physical and chemical processes in nature.

In Manchester we also built in 1999 the first of a new breed of instruments capable of analysing samples as small as a few hundred atoms. The results of a recent measurement reported in the research journal Science show the presence of the isotope xenon-129, produced from the radioactive decay of a now-extinct isotope, iodine-129, which was itself produced in an exploding star. Found in tiny grains of halite (rock salt) in a primitive meteorite along with minute inclusions of water, it provides evidence that liquid water, a critical component of life, was flowing through the precursors of the planets within 2 million years of the Solar System’s birth.

The most remarkable discovery in recent years has been the isolation of ‘stardust’. These are minute grains of diamond, silicon carbide, graphite and corundum (aluminium oxide) which condensed in the atmospheres of stars, millions of years before the birth of our Solar System. In their bizarre isotopic signatures they carry a story of how the chemical elements, which came together to make up the Earth and ourselves, were generated by nuclear processes in the interiors of long dead stars. Current methods for isolating these ‘pre-solar grains’ are decidedly crude and involve dissolving most of the meteorite in strong acids. The process is sometimes described as ‘burning down the haystack to find the needle’ and raises the question, still to be answered, of what information is being lost with the haystack!

The first extraterrestrial samples to be returned to Earth, since Apollo, will arrive later this decade in 2006. These will be grains of dust collected by the NASA Stardust mission from the comet Wild. Comets are frozen relics of the material which accumulated to form the Solar System 4570 million years ago, a time capsule of our own beginnings. Captured in aerogel, a man-made silica-based solid barely 10 times as dense as the air we breath, the sample will consist of several thousand grains less than one-tenth of a millimetre across. Their analysis will demand the development of a new breed of analytical instrument with unpreced-ented sensitivity at the atomic scale.

Sometime before 2010, samples of Mars will also be returned to Earth and be subjected to the barrage of techniques, which only the vast array of laboratory-based equipment makes possible. Comparison with the effect of the lunar sample programme in the 1970s suggests that the critical technologies developed to analyse these unique samples, and those of cometary grains, which precede them in 2006, will have a major cross-fertilization into analytical instrumentation in other fields.

Much has changed since Sorby’s time. Optical microscopes, electron microscopes and many other kinds of sophisticated instrument are now essential tools in fields as diverse as earth and planetary science, semiconductor research, forensic science, nanotechnology, the micro-environment, and so on. But some aspects remain the same – the complex interplay of fundamental science and technology continue to enhance our lives.

Professor G.Turner is Professor of Isotope Geochemistry and head of the Cosmochemistry Research Group at the University of Manchester. (This feature is based on an article that appeared in Science and Parliament)